The 20/21 voices are then added in 'random' phase, and changed frequently: by fading-out one while simultaneously fading-in another [at another sample position] using, for example, a cosine quadrant multiplier - which is process-time costly, so it may be done with one in-out-pair of voices at each moment: on the order of the half-phoneme interval. [30msec quadrant => 120msec wavelength => 8Hz pitch variation at transition - deemed unheard at 8Hz=3%, meaning above 250Hz-voices, or above 12Hz for one voice at a time. Nota Bene: this is a suggested musical estimate, but shows the possibility with ample margin]
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